We have fabricated a Si racetrack optical modulator based on a III–V/Si hybrid metal-oxide-semiconductor (MOS) capacitor. The III–V/Si hybrid MOS optical phase shifter was integrated to a Si racetrack resonator with a coupling length of 200 µm and a coupling gap of 700 nm. The fabricated Si racetrack resonator demonstrated a small V_πL of 0.059 Vcm. For 10-dB optical intensity modulation, the Si racetrack resonator showed a 60% smaller driving voltage than a Mach–Zehnder interferometer modulator with the same phase shifter, leading to a better balance between high energy efficiency and large modulation bandwidth.